(1) Field of the Invention
The present invention relates generally to pre-wound sewing machine bobbins and, more particularly, to a detectable bobbin and core for use in critical applications such as manufacturing air bags.
(2) Description of the Prior Art
A lockstitch sewing machine requires the use of a bobbin to provide the underthread which interlaces with the top thread supplied by the sewing needle to produce a seam. Historically these bobbins have been wound at the sewing machine on metal shells initially supplied by the sewing machine manufacturer. These self-wound bobbins typically do not provide the highest productivity and quality demanded by the sewn products industry. Specifically, it takes more time to hand wind a bobbin than to use a pre-wound bobbin and a handwound bobbin usually only holds about 1/2 the amount of thread as a pre-wound bobbin due to better packing using automatic machinery.
These pre-wound bobbins are normally composed of an appropriate sewing thread wound on a plastic or paper cylindrical core with overall dimensions (i.e., thickness, inside diameter and outside diameter) required by a given sewing machine. Manufacturing of a pre-wound bobbin requires a sturdy core with a frictional surface that ensures correct and dependable windability. Pre-wound bobbins provide higher quality and productivity by virtue of greater thread capacity, uniform thread lengths and reproducible pull-off tension compared to self-wound bobbins.
In some applications (e.g., air bag manufacturing), there is a need to ensure that there are no extraneous materials inadvertently left inside the completed assembly. To ensure that an accidently misplaced bobbin or bobbin core is found and removed, there is a need for these items to be detectable by non-destructible means. Current versions of pre-wound bobbins and associated bobbin cores are unacceptable for these applications because they cannot be detected.
One unacceptable method for achieving this detection capability is to self-wind bobbins utilizing the original metal shell provided with the sewing machine. Although easily detected, these self-wound bobbins yield lower productivity and quality than required as discussed above. Additionally, there is a very high liability risk associated with a metal shell that is accidentally undetected. In the case of an air bag, the weight of the metal shell is sufficient to produce a very dangerous projectile with high kinetic energy under the rapid deployment of the air bag.
Thus, there remains a need for a new and improved bobbin for a sewing machine which may be easily detected if accidentally left in the sewn article, such as an air bag while, at the same time, can be easily pre-wound using automatic equipment.